A common technique to locate a terminal is to determine the amount of time required for signals transmitted from multiple transmitters at known locations to reach the terminal. One system that provides signals from a plurality of transmitters at known locations is the well-known Global Positioning Satellite (GPS) system. Satellites in the GPS system are placed in precise orbits according to a GPS master plan. The position of the GPS satellites can be determined by different sets of information (commonly known as the “Almanac” and “Ephemeris”) transmitted by the satellites themselves. Another system that provides signals from transmitters (e.g., base stations) at known earth-bound locations is a wireless (e.g., cellular telephone) communication system.
Many wireless communication systems employ repeaters to provide coverage for designated areas within the system or to extend the coverage of the system. For example, a repeater may be used to cover a particular area not covered by a base station due to fading conditions (i.e., a “hole” within the system). Repeaters may also be used to extend coverage into rural areas (e.g., along a freeway) that are outside the coverage area of the base stations. A repeater receives, conditions, and retransmits signals on both the forward link (i.e., the path from the base station to the mobile unit) and reverse link (i.e., the path from the mobile unit to the base station).
Various challenges are encountered in determining the location of a terminal in a system in which one or more repeaters are employed. Typically, a signal from a single base station is processed and retransmitted by a repeater at relatively high power and with a delay. The combination of the repeated signal's high power plus the isolation normally associated with the repeater's coverage area often prevent a terminal from receiving other signals from other base stations. Moreover, in many cases in which repeaters are used (e.g., inside buildings, tunnels, subways, and so on), the signals from GPS satellites have insufficient power levels to be received by the terminal. In this case, a limited number of signals (possibly only one signal, from the repeater) may be available for use to determine the terminal's location. Furthermore, the additional delays introduced by repeaters can distort the round trip delay/time of arrival (RTD/TOA) measurements as well as the TDOA measurements, which then results in inaccurate position estimate based on these measurements.
FIG. 1A is a diagram of a wireless communication system 100 that employs repeaters in accordance with the disclosed method and apparatus. System 100 may be designed to conform to one or more commonly known industry standards, such as IS-95, published by the Telecommunications Industry Association/Electronics Industry Association (TIA/EIA), and other such industry standards for systems such as W-CDMA, cdma2000, or a combination thereof. System 100 includes a number of base stations 104. Each base station serves a particular coverage area 102. While only three base stations 104a through 104c are shown in FIG. 1A for simplicity, it will be understood by those skilled in the art that there are typically many more such base stations in such a system. For the purpose of this disclosure, the base station and its coverage area are collectively referred to as a “cell”.
One or more repeaters 114 may be employed by system 100 to provide coverage for regions that would not otherwise be covered by a base station (e.g., due to fading conditions, such as region 112a shown in FIG. 1A) or to extend the coverage of the system (such as regions 112b and 112c). For example, repeaters are commonly used to improve indoor coverage for a cellular system at relatively low costs. Each repeater 114 couples to a “serving” base station 104 via a wireless or wireline link (e.g., a coaxial or fiber optic cable) either directly or through another repeater. Any number of base stations within the system may be repeated, depending on the particular system design.
A number of terminals 106 are typically dispersed throughout the system (only one terminal is shown in FIG. 1A for simplicity). Each terminal 106 may communicate with one or more base stations on the forward and reverse links at any moment, depending on whether or not soft handoff is supported by the system and whether or not the terminal is actually in soft handoff. It will be understood by those skilled in the art that “soft handoff” refers to a condition in which a terminal is in communication with more than one base station at the same time.
A number of base stations 104 are typically coupled to one base station controller (BSC) 120. BSC 120 coordinates the communication for base stations 104. For the purpose of determining the position of terminal, base station controller 120 may also be coupled to a Position Determining Entity (PDE) 130. PDE 130 receives time measurements and/or identification codes from the terminals and provides control and other information related to position determination, as described in further detail below.
For position determination, a terminal may measure the arrival times of signal transmissions from a number of base stations. For a CDMA network, these arrival times can be determined from the phases of the pseudo-noise (PN) codes used by the base stations to spread their data prior to transmission to the terminals over the forward link. The PN phases detected by a terminal may then be reported to the PDE (e.g., via IS-801 signaling). The PDE then uses the reported PN phase measurements to determine pseudo-ranges, which are then used to determine the position of the terminal.
The position of a terminal may also be determined using a hybrid scheme whereby signal arrival times (i.e., times of arrival (TOA)) are measured for any combination of base stations 104 and Global Positioning System (GPS) satellites 124. The measurements derived from GPS satellites may be used as the primary measurements or to supplement the measurements derived from the base stations. The measurements from the GPS satellites are typically more accurate than those from the base stations. However, clear line-of-sight to the satellites is typically required to receive the GPS signals. Accordingly, the use of GPS satellites for position determination is generally limited to outdoor environment where obstructions are not present. GPS signals typically cannot be received indoors or in other environments where there are obstructions such as foliage or buildings. However, GPS has extensive coverage and four or more GPS satellites can potentially be received from virtually anywhere that there are no such obstructions.
In contrast, base stations are typically located in populated areas and their signals are able to penetrate some buildings and obstructions. Therefore, it is possible for base stations to be used in cities and potentially within buildings to determine the location of devices that can receive and/or transmit such signals. However, the measurements derived from base stations are typically less accurate than those from GPS satellites because multiple signals may be received at the terminal from a particular base station due to a phenomenon known as “multipath”. Multipath refers to the situation in which signals are received via multiple transmission paths between the transmitter and receiver. Such multiple paths are generated by signals reflecting off various objects, such as buildings, mountains, etc. It should be noted that in the best case, the signal is also received on a direct path (straight line) from the transmitter to the receiver. However, this may not necessarily be true.
In the hybrid scheme, each base station and each GPS satellite represents a transmission source. To determine a two-dimensional estimate of the position of a terminal, the transmissions from three or more non-spatially aligned sources are received and processed. A fourth source may be used to provide altitude (a third dimension) and may also provide increased accuracy (i.e., reduced uncertainty in the measured arrival times). The signal arrival times can be determined for the transmission sources and used to compute pseudo-ranges, which can then be used (e.g., via a trilateration technique) to determine the position of the terminal. Position determination can be achieved by well know means, such as is described in the 3GPP 25.305, TIA/EIA/IS-801, and TIA/EIA/IS-817 standard documents.
In the example shown in FIG. 1A, terminal 106 may receive transmissions from GPS satellites 124, base stations 104, and/or repeater 114. Terminal 106 measures the signal arrival times of the transmissions from these transmitters and may report these measurements to PDE 130 via BSC 120. PDE 130 can then use the measurements to determine the position of terminal 106.
As noted above, repeaters may be used to provide coverage for regions not covered by the base stations, such as within buildings. Repeaters are more cost effective than base stations, and can be advantageously deployed where additional capacity is not required. However, a repeater is associated with additional delays due to circuitry within the repeater and cabling and/or additional transmission associated with the repeater. As an example, surface acoustic wave (SAW) filters, amplifiers, and other components within the repeater introduce additional delays that are comparable to, or may be even greater than, the transmission delays from the base station to the terminal. If the repeater delays are not taken into account, then the time measurements of the signals from repeaters cannot be reliably used to determine the position of a terminal.
FIG. 1B is a diagram illustrating the use of a repeater 114x to provide indoor coverage for a building 150. In the example shown, repeater 114x comprises a main unit (MU) 115 coupled to a number of remote units (RUs) 116. On the forward link, main unit 115 receives one or more signals from one or more base stations and repeats all or a subset of the received signals to each of the remote units. And on the reverse link, main unit 115 receives, combines, and repeats the signals from remote units 116 for transmission on the reverse link back to one or more base stations. Each remote unit 116 provides coverage for a particular area (e.g., one floor) of the building and repeats the forward and reverse link signals for its coverage area.
Various challenges are encountered in estimating the position of a terminal located within a building where a repeater may be employed to provide coverage. First, in many indoor applications, the terminals are not able to receive signals from the base stations or GPS satellites, or may receive signals from fewer transmitters than required to perform trilateration. To provide in-building coverage, a repeater typically retransmits a signal from a single base station at relatively high power and with a delay. The combination of the repeated signal's high power plus the isolated indoor location of the terminal normally prevent the terminal from receiving other signals from other base stations and satellites.
Second, if the amount of delay introduced by the repeater is not known, then the signal from the repeater cannot be reliably used as one of the signals for trilateration. This then prevents an entity (e.g., the PDE or terminal) from utilizing the repeated signal to derive a positioning estimate with one less satellite or base station signal. Third, in many environments where repeaters are used (e.g., subways, buildings, and so on) GPS signals cannot be received, even when a terminal employs a receiver unit with enhanced sensitivity. And fourth, the entity used to determine the terminal's position has no way of determining whether the terminal was using an incorrect timing reference (due to the uncertain repeater delay), which would affect the accuracy of the round trip delay (RTD) measurements and the time stamp on the GPS measurements.
There is therefore a need in the art for techniques to provide a position estimate of a terminal in a wireless communication system that employs repeaters (or other transmission sources with similar characteristics).